Objective, quantitative and practicable measures of psychological stress are pivotal to studies in many branches of vertebrate biology, including wildlife conservation and management, and are also relevant to the farming industry, animal welfare and veterinary services. Techniques for quantifying levels of human stress are important from a medical perspective, but are also of interest for occupational safety and health. An estimated 1 in 10 people in the UK suffer from work related stress (Health & Safety Executive Information sheet: 1/01/EMSU updated February 2002) at a cost of £3.7 billion to society. Stress-related conditions or effects include immune disorders, cardiovascular disease, muscoskeletal and psychological disorders, workplace injury and ulcers.
Various methods have previously been applied in an attempt to assess stress responses, including the perceived stress questionnaire (Cohen et al., J. Health & Social Behaviour (1983) 24, 385-396), measurement of cortisol and hormonal levels in blood, urine, saliva and faeces (Beerda et al., Horm. Behav. (1996) 30, 272-279), haematological values (e.g. Millspaugh et al., Can. Field-Nat. (2000) 114, 196-200) and behavioural observations (reviewed by Rushen in The Biology of Animal Stress Basic Principles And Implications For Animal Welfare (2000) ed. Moberg & Mench, Wallingford:CABI Publishing). However, these approaches have various drawbacks, and may be inappropriate to use in certain situations. The perceived stress questionnaire is subjective, and so cannot provide an objective measure of stress. Furthermore, questions may not always be answered honestly, for example due to cultural reasons, and this measure of stress cannot be used for testing animals or birds. Measurements of cortisol and hormonal levels in, for example, blood and urine, are often used to assess stress in animals, but these measures are not ideal since their levels change throughout the year. There are no absolute terms of reference, and this type of testing is time consuming. Behavioural observations are also often subjective and difficult to quantify.
More recently, it was shown that exposure of human volunteers to a short term mental stressor leads to increased superoxide anion production being observable in neutrophils present in peripheral blood samples (Ellard et al., Inter. J. Psychophys. (2001) 41, 93-100). It has now been found that exposure of both animals and humans to psychological stress can be rapidly and readily quantified by relying on measurement of the retained capacity of neutrophils in peripheral blood samples to produce superoxide anions in response to a challenge by phorbol myristate acetate (PMA), a known chemical-inducer for activating neutrophils (Hu et al., Cell Signal (1999) 11, 355-360). By this means, for the first time, it has proved possible to obtain a quantitative measurement of copying capacity of animals and humans for known or suspected psychological stressors.